Regulated D.C. power supplies provide predictable and reliable voltage sources for driving electronic circuitry. The conventional power supply design typically employs a power device for developing a D.C. voltage output and a regulating feedback loop. The regulation loop serves to maintain the power supply output voltage at a pre-selected set point by sensing the output voltage and increasing or decreasing the output relative to the desired set point.
To dampen the response of the feedback loop by changing the supply output voltage from 0 volts to a preselected set point voltage relatively slowly, rather than instantaneously, many power supply designs employ a slew rate limiting circuit. The slew rate circuit tends to reduce the stress on any loads developed by the sudden application of power to a deenergized electronic circuit.
The dampening effect of the slew rate circuit also reduces any transient voltage overshoot associated with the power supply regulation loop. Overshoots often develop from the fast feedback response of the system that produces a high initial error at start-up, causing saturation of the control loop with a corresponding overshoot above the desired set point.
Conventional slew rate limiting circuits typically fall into two categories: rampable set points and modulation limiters. Rampable set point circuits are constructed to change the desired output voltage from 0 volts to the desired value smoothly over time. U.S. Pat. No. 4,598,351 illustrates a typical rampable set point design that includes a reference capacitor coupled in parallel with a zener diode. A constant current source charges the capacitor at start-up to produce a ramping reference voltage fed to the input of an error amplifier. The reference voltage is clamped to a maximum value by the zener diode.
While the rampable set point design works well for its intended applications, the rate at which the capacitor charges to ramp the voltage up is not easily changed. This is because of the discrete component design that minimizes any controllable variation in the rate. Moreover, no provisions are included for changing the set point for the circuit in a predictable, linear manner.
In contrast to the rampable set point construction, modulation limiter designs generally include a grounded capacitor coupled to a forward-biased diode disposed at the input of an error amplifier. The error amplifier is employed in a feedback loop to effect voltage regulation. A second diode is disposed in parallel to the dioded capacitor and selectively couples a normal control signal to the amplifier input node.
In operation, the diode branch with the lowest input voltage to the amplifier sets the control signal. At start-up, the lowest voltage is at the dioded capacitor, which charges up to produce an increasing voltage until the capacitor diode reverse biases, at which time the second diode forward biases, enabling the normal control signal to set the output voltage.
Although modulation limiter circuits perform well for their intended applications, they suffer from many of the problems plaguing conventional rampable designs. Again, because of the arrangements of the discrete components, the slew rate is often non-adjustable. Additionally, typical modulation limiter circuits fail to include circuitry to vary the desired set points.
Therefore, the need exists for a power supply having a slew rate limiting circuit that provides control capability not only for the slew rate, but additionally offers the capability of adjusting the set point in a predictable linear manner. The power supply and method of the present invention satisfies these needs.